From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
In about 1975 superconductivity was discovered in a member of a new class of materials, namely metal oxides. See, for instance, A. W. Sleight et al, Solid State Communications, Vol. 17, page 27 (1975), and U.S. Pat. No. 3,932,315. The bismuth lead oxides of the paper and the '315 patent have crystal structures that are closely related to the well-known perovskite structure (having an ABO.sub.3 structure, in which A and B designate appropriate metal elements, with the so-called A-site being crystallographically inequivalent to the B-site) with mixed B-site occupancy. These oxides typically have composition BaPb.sub.1-x Bi.sub.x O.sub.3, with 0.05.ltoreq.x.ltoreq.0.3, and become superconducting at temperatures up to about 13K, T.sub.c typically depending on the composition of the material. The maximum T.sub.c is observed for x.about.0.25. A metal/semiconductor transition occurs for x.about.0.35, with the semiconducting behavior continuing to the end member compound BaBiO.sub.3.
In addition to the Pb-rich bismuth oxides (both Pb and Bi located on B-sites) of the Sleight et al., article, the '315 patent also discloses superconducting Pb-Bi-oxides having, in addition to mixed B-site occupancy, mixed A-site occupancy. However, to the best of applicants' knowledge, no prior art Bi-based (copper-free) oxide superconductor has exhibited a T.sub.c .gtorsim.13K.
In 1986, J. G. Bednorz and K. A. Muller, Zeitschr.f.Physik B-Condensed Matter, Vol. 64, 189, reported the discovery of superconductivity in lanthanum barium copper oxide. This report stimulated worldwide research activity which very quickly resulted in further significant progress.
The progress has resulted, inter alia, to date in the discovery that compositions in the Y--Ba--Cu--O system can have T.sub.c above 77K, the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al., ibid, page 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of compositions and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c of about 90K.
A new class of copper-based oxide superconductors was recently discovered almost simultaneously in Japan and the USA, by groups lead by H. Maeda and C. W. Chu, respectively. See also, M. A. Subramaniam et al, Science, Vol 239, pages 1015-1017, Feb. 26, 1988, and U.S. patent application Ser. No. 155,330. The recently discovered Bi--Sr--Ca--copper oxide has a transition temperature of about 80K, with a phase that has a substantially higher transition temperature frequently being present in samples of the material.
As indicated by the above brief review, most presently known oxide superconductors are either copper oxides or bismuth oxides. All of the prior art Bi-oxide superconductors have perovskite-like structure, with mixed occupancy on the B-site.
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be perhaps the most advantageous cryogenic refrigerant, and attainment of superconductivity at liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
Although this goal has now been attained, there still exists scientific as well as technological interest in the discovery of novel superconductive oxides. Such compounds would, of course, be of immense help in the elucidation of the mechanism responsible for the high transition temperatures observed in some of the oxide superconductors. Furthermore, among such novel compounds may be some that have a relatively high transition temperature and show improvement in one or more properties relative to currently known high T.sub.c materials. For instance, many of the currently known high T.sub.c superconductors have in bulk form relatively low current carrying capacity, especially in the presence of a magnetic field. These materials also tend to be brittle and at least some compounds (e.g., the well-known "1--2--3" compound YBa.sub.2 Cu.sub.3 O.sub.7) are relatively unstable in the presence of water vapor, CO, etc. Consequently, an intense effort has been underway worldwide to discover new superconductive oxides. This application discloses a new class of such oxides.
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDs and Machines, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets, for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if oxidic superconductive material could be used instead of the previously considered or used superconductors.